The present invention relates to an electrophotographic color image forming apparatus wherein a plurality of imagewise exposure means and developing means are arranged along the circumferential surface of an image-forming object that is mainly in a drum shape, and toner images are formed and superimposed while the image-forming object is rotated within one turn.
As a method for forming a multi-color image, there have been known some methods including apparatus (A) wherein photoreceptors, charging units and developing units each in quantity equivalent to the number of colors necessary for the multi-color image are provided, and toner images each being a mono-color formed on each photoreceptor are superimposed on an intermediate transfer object to form a color image, apparatus (B) wherein one photoreceptor is caused to make plural turns so that charging, imagewise exposure and developing for each color are repeated for forming a color image, and apparatus (C) wherein charging, imagewise exposure and developing for each color are conducted in succession while one photoreceptor makes one turn for forming a color image.
However, the apparatus (A) has a drawback that the apparatus is destined to be large in size because a plurality of photoreceptors and intermediate transfer objects are required, while the apparatus (B) has a restriction that a size of an image formed is limited to the surface area or less of the photoreceptor though the apparatus can be small in size because the required number of each of the charging means, imagewise exposure means and photoreceptor is just one.
In the case of the apparatus (C), although it makes it possible to form images at high speed, it still has a contradiction that a diameter of a photoreceptor is large and thereby the apparatus is also large due to the following two reasons; one is necessity that a plurality of charging units, imagewise exposure means and developing units need to be arranged within a circumferential surface of the photoreceptor, and the other is necessity that the distance between the imagewise exposure means and the developing unit needs to be large for avoiding a fear that image quality is deteriorated by toner leaking from the developing unit to which an imagewise exposure optics system is close.
For the purpose of avoiding a drawback of the aforementioned contradiction in the apparatus (C), there has been suggested an apparatus wherein the base of an image-forming object is formed with a transparent material, a plurality of imagewise exposure means are housed inside the image-forming object, and a light-sensitive layer formed on the external surface of the base is exposed to light reflected on an image through the base (for example, Japanese Patent Publication Open to Public Inspection No. 307307/1993 (hereinafter referred to as Japanese Patent O.P.I Publication)).
However, the apparatus in accordance with the aforementioned suggestion has disadvantages including complicated structure due to arrangement of many imagewise exposure means inside the image-forming object and many charging units and developing units outside the image-forming object, inefficient handling due to complicated mounting and dismounting of developing units, image-forming objects and imagewise exposure means, and difficulty of keeping positional accuracy between various units. In particular, it is difficult to provide an optics system fixed inside the image-forming object and to rotate it or to mount and dismount it. It is further preferable that a common structure is used for various developing units.
It is further difficult on the apparatus in accordance with the aforementioned suggestion that optics systems such as LED, LCS, PLZT, LISA and others are arranged inside the image-forming object and the image-forming object is rotated, or mounted and dismounted.
With regard to the exposure optics system, in particular, positional relationship between various parts in the optics system and that between the optics system and the image-forming object are kept to be highly accurate. Therefore, when deformation or displacement is caused by the mounting or dismounting of the image-forming object, registration or an image forming position is changed, resulting in inability to obtain excellent color images. It is preferable to integrate developing units and others to the utmost.
Further, under a certain layout of those units, a toner hopper can be built in without providing any large space in particular, and it is easy to connect it to or integrate it into developing units.
Furthermore, in the aforementioned apparatus (C), it is possible to use an image-forming object having a small diameter due to an imagewise exposure means housed therein, and it is possible to make volume of the apparatus small. However, when incorporating a duplex copy function, the apparatus is required to be high and its compactness is lost because it is necessary to incorporate a known sheet reversing and feeding path which reverses a transfer material and feeds it to the transfer section again.
There is further a problem that the length of sheet feeding path is increased and thereby frequency of jam occurrence is enhanced.